1. Field of the Invention
The present invention relates to electrochemical processes, and particularly to an electrochemical method of producing hydrogen peroxide using a titanium oxide nanotube catalyst that uses an electrode with TiO2 nanotubes annealed with nitrogen for the production of hydrogen peroxide by the electrochemical reduction of oxygen in an alkaline solution.
2. Description of the Related Art
Hydrogen peroxide (H2O2) is an environmentally-friendly oxidizing agent which is extensively used in environmental disinfection and in the chemical industry as an oxidizing chemical reagent, a bleaching agent for textiles and paper products, and for cleaning, stripping and etching processes in the semiconductor industry. Water is the sole degradation product of hydrogen peroxide, thus making it extremely desirable from an environmental standpoint.
The industrial production of hydrogen peroxide is typically performed by an anthraquinone reduction reaction (typically referred to as the “AQ process”), which involves the hydrogenation of anthraquinone derivatives (with a catalyst, such as nickel or palladium), which produces the corresponding hydroquinones, followed by oxidation with oxygen (typically from air) to yield hydrogen peroxide and reproduce the initial anthraquinone. The AQ process, however, involves successive hydrogenation and oxidation of an alkyl-anthraquinone precursor, which is dissolved in a mixture of organic solvents, followed by liquid-liquid extraction and recovery of H2O2. This multistep method consumes significant energy input and generates large amounts of waste. Additionally, the H2O2 product is often contaminated with organic waste and byproducts, which must be constantly removed throughout the process. Thus, a more efficient method for the direct synthesis of H2O2 from oxygen (O2) would obviously be desirable.
Other methods used for the production of H2O2 include electrochemical oxygen reduction and direct synthesis from hydrogen and oxygen in the presence of noble metal catalysts supported on silica, alumina or carbon. The only viable electrolytic H2O2 production process that competes with the AQ process is the Dow process, which was developed for on-site site production of H2O2 by cathodic reduction of oxygen using a graphite electrode in a trickle bed cell. The H2O2 solution produced by the process is directly used in pulp bleaching and in the processing of recycled paper. The production of H2O2 relies on the electrolysis of O2 in alkaline solutions in an electrochemical cell using a carbon cathode, and the electrochemical reactions at the anode and cathode are respectively given by:2OH−→H2O+½O2+2e−  (1)H2O+O2+2e−→HO2−+OH−  (2)The evolution of oxygen at the anode in reaction (1) is very beneficial, as it can diffuse through a flow electrolyte and be reduced at the cathode, as shown in reaction (2), to produce H2O2 (i.e., a self-feeding process). The cathode in the Dow process is made from graphite coated with carbon black and a fluorocarbon binder to facilitate O2 transfer at atmospheric pressure. A cell operating at 2.3 V and 62 mA/cm2 yields NaOH/HO2− at a weight ratio of 1.6-1.8 to 1 and at a current efficiency of 90%. This alkali peroxide technology is well suited to bleaching applications, where it is not necessary to separate the peroxide from the caustic soda in the product.
FIG. 9 illustrates a typical prior art electrochemical cell 1 for performing such reactions as the Dow process. The cell 1 includes a central cathode structure 3 and an outer, surrounding anode 4, which is supported on the interior of the cell 1. An alkaline electrolyte 5 is retained by the cell 1 between the anode 4 and the exterior surface 6 of cathode 3. The anode 4 and cathode 3 are separated by a cylindrical diaphragm 7. Electrical leads 8, 9 to the anode 4 and cathode 3, respectively, provide electrical energy to the system. A cooling coil 10 is often employed in such reaction cells.
The cathode 3 is of generally cylindrical shape and includes a lead-in conduit 11 for carrying oxygen or oxygen-containing gases to the cathode interior, where it travels to the exterior face 6 of the cathode 3 through the porous body 12 of the cathode. The conduit 11 and the porous body 12 are electrically conductive. The cathode body 12 is of porous graphite and will pass the necessary oxygen-containing gas constituent.
Photocatalytic formation methods of H2O2 over semiconductor oxides have also been investigated in recent years. Reactive oxygen-containing species (ROS), namely OH−, O2−, and H2O2, are usually formed at the surface of titanium oxide (TiO2) under UV irradiation. TiO2 anatase and rutile crystalline forms as photoactive catalysts have shown different reactivities for the photo-production of H2O2. The anatase is produced at a lower temperature than the rutile phase and has shown higher photo-activity due to the presence of a higher density of surface defects, such as oxygen vacancies and sub-oxides, than the rutile form, which contributes to the higher catalytic activity observed for the production of hydrogen peroxide on anatase powder. However, the concentrations of H2O2 produced using photo-irradiated TiO2 surfaces are in the micromolar range, which does not meet the requirements for industrial scale production.
TiO2 nanotube (TON) catalysts are self-assembled via anodization and have attracted considerable interest in recent years due to their unique nanoscale features and electronic properties. TON has unique semiconducting properties, chemical inertness and stability, is cost effective and resistant to corrosion, and has applications in numerous fields, such as photocatalysis, solar cells, electronic devices, and environmental cleaning and protection. TON arrays prepared by anodization typically exhibit relatively low electrical conductivity, which limits applications in electro-catalysis and use as a catalyst support. Thus, various methods for generating oxygen vacancies, such as metal and non-metal doping, are often employed to improve the electrical conductivity and reactivity of TON structures to satisfy the requirements for effective electrode materials.
The oxygen reduction reaction (ORR) in aqueous solutions occurs primarily through two pathways, the direct 4-electron reduction pathway from O2 to H2O, and the 2-electron reduction pathway from O2 to hydrogen peroxide (H2O2). In non-aqueous aprotic solvents and/or in alkaline solutions, the 1-electron reduction pathway from O2 to superoxide (O2−) can also occur. The electrochemical reduction of oxygen (ORR) has been found to occur on various forms of plain titanium dioxide at a much higher overvoltage in acid and alkaline media. It has also been found that the overvoltage for ORR is significantly reduced after activation of the TiO2 layer by cyclic polarization. However, despite recent experiments, such methods have yet to achieve the effective production of hydrogen peroxide on TON arrays in alkaline media.
Thus, an electrochemical method of producing hydrogen peroxide using a titanium oxide nanotube catalyst solving the aforementioned problems is desired.